1. Field of the Invention
The invention is related to the field of ion mobility spectrometers and in particular to an ion mobility spectrometer having means for continuously adding a chemical reagent vapor to the carrier gas in order to improve the selectivity of the spectrometer and eliminate the adverse affects of interferring vapors.
2. Prior Art
Ion mobility spectrometers are the primary instrument used in the field of plasma chromatography. The operation of the ion mobility spectrometer is similar to the operation of a time-of-flight mass spectrometer, the difference being that a time-of-flight spectrometer operates in a vacuum where the mean free path of the contained gases is may times the dimensions of the gas container whereas the ion mobility spectrometer operates at or near atmospheric pressure where the mean free path of the contained gas is smaller than the dimensions of the container. A typical ion mobility spectrometer, such as disclosed by Cohen et al, U.S. Pat. No. 3,621,240, comprises an ion/molecule reaction chamber, an ionization source associated with the ion reaction chamber, an ion drift chamber, an ion/molecule injector shutter or grid interposed between the ion reaction chamber and the ion drift chamber, and an ion collector. A carrier gas, normally purified air or nitrogen, is introduced into the ion mobility spectrometer to transport vapor from a sample material to be identified or detected. The carrier gas with the sample vapor are ionized by the ionization source in the ion/molecule reaction chamber. However, as is known in the art, the relative concentrations of the carrier gas and the sample vapor are such that the molecules of the carrier gas are more easily directly ionized by the ionization source than the molecules of the sample vapor. Since the mean free path for the ionized carrier molecules is many times smaller than the dimensions of the reaction chamber, there are multiple collisions between the ionized carrier molecules and the sample molecules. The tendency of these collisions is to transfer the ion charge from the ionized carrier molecules to the sample molecules. Therefore, the ionization of the sample gas is primarily by this secondary ionization process.
The charged molecules or ions are accelerated to a terminal velocity by an electrostatic field gradient within the ion/molecule reaction chamber causing them to travel towards the injection grid interfacing the ion drift chamber. Periodically, the bias on the injection grid is reduced to zero for a short period of time to permit a quantity (pulse) of ions to pass from the ion/molecule reaction chamber to the ion drift chamber. In the drift chamber, the passed ions are drawn, under the influence of an electrostatic drift field, to the ion collector where they are collected. The time of arrival of each ion species, both carrier gas and sample, at the ion collector is determined by the particular ion's mobility through the non-ionized gas filling the drift chamber. The heavier ions characteristically move slower through the drift chamber and arrive at the ion collector later than the lighter ions. It is thus possible to characterize the different ion species by monitoring the time between the introduction of the ions into the drift chamber at the injection grid and the arrival of the ions at the ion collector. An electrometer measures the quantity of ions collected by the ion collector.
As is known in the art, the sample vapor is injected into a carrier gas which transports the vapor molecules to the ion/molecule reaction chamber and exposes the vapor molecules to ion/molecule reactions with the ions generated in the ionized carrier gas. If it is desired to determine whether the atmosphere contains a certain constituent, usually a contaminant, the sample can simply be a sample of ambient air. However, certain constituents of atmospheric air, such as water, ammonia and nitrogen oxides, interfere with the proper performance of the ion mobility spectrometer. The effects of these components can be significantly attenuated through the use of a membrane inlet filter described in U.S. Pat. No. 4,311,669 issued Jan. 19, 1982. However, additional improvements in specificity are needed to allow ion mobility spectrometry to be used as a detector for specific sample materials in the presence of contaminants or other interferences.
One such improvement is described by Munson in U.S. Pat. No. 3,555,272. Munson discloses a method for generating sample ions in a mass spectrometer by mixing a reagent gas with the sample vapors. When the mixture is admitted into the ionization chamber of an ion source, the reagent gas is ionized first to form stable ions. The stable ions of the reagent gas then undergo ion/molecule reactions with the sample vapor to form ions characteristic of the sample vapor. The concentration of the sample in the ionized reagent gas is less than 1 percent. Similarly, Anbar et al in U.S. Pat. No. 3,920,987 disclose a method for detecting explosives in which a reagent gas comprising a mixture of sulfur hexafluoride (SF.sub.6) and nitrogen (N.sub.2) is first ionized in a separate chamber to form stable negative ions. The ionized reagent gas is then mixed with the sample vapor in an atmospheric pressure reaction chamber. In the atmospheric pressure reaction chamber electron exchange reactions occur to ionize the explosive constituent of the sample, before the ions are transmitted to a mass analyzer at reduced pressure for analysis. Sulfur hexafluoride (SF.sub.6) was selected because it had a lower electron affinity than the explosive molecules in the sample but a higher electron affinity than the other constituents in the air which serves as the carrier for the explosive samples.
Finally, McClure in U.S. Pat. No. 4,374,090 discloses a method for detecting certain chemical agents in which a reagent gas of DMMP (dimethymethylphosphonate), TBA tributylamine, DIMP (diisopropylmethylphosphonate), DMSO (dimethylsulfoxide), di-N-butylamine and mixtures thereof are first ionized in the reaction chamber of an ionization detector as described in U.S. Pat. No. 3,835,328. The stable ions formed from the reagent gas then undergo ion/molecule reactions with the sample vapor to produce ions characteristic of the sample vapor which can be analyzed by the ionization detector.
Positive ion or proton transfer reactions are among the most important class of ion/molecule reactions used in ion mobility spectrometry. If M is the sample molecule and RH.sup.+ is the reactant ion, ionization occurs in accordance with: EQU RH.sup.+ +M.fwdarw.R+MH.sup.+
where MH.sup.+ is the product ion. The tendency of the proton H.sup.+, to transfer from the reactant ion RH.sup.+ to the sample M is regulated by the relative proton affinities of R and M. The proton affinity of M must be greater than the proton affinity of R if the proton transfer reaction is to occur.
For the (H.sub.2 O).sub.n H.sup.+ reactant ion conventionally used in ion mobility spectrometry, it has been shown that organophosphorous compounds, such as dimethylmethylphosphonate (DMMP), are ionized according to: EQU (H.sub.2 O).sub.n H.sup.+ +DMMP.fwdarw.(DMMP)H.sup.+ +nH.sub.2 O
when n is small, i.e. n=4 or 5.
Additionally, cluster reactions can take place to yield: EQU M+MH.sup.+ .revreaction.M.sub.2 H.sup.+
where M.sub.2 H.sup.+ is the dimer ion.
As a result of the low proton affinity (173 Kcal/mole) of the (H.sub.2 O).sub.n H.sup.+ reactant ion and its strong tendency to change cluster size with small variations in water concentration, two difficulties are encountered in its use as a reactant ion. First, the (H.sub.2 O).sub.n H.sup.+ reactant ion loses charge to a large number of compounds, such as alcohols, ketones, aldehydes, esters, amines, pyridines, etc. which have proton affinities greater than itself and second, its cluster size changes with water concentration causing it to shift in drift time in the ion mobility spectrum. For a microprocessor controlled ion mobility spectrometer system, it is desirable to eliminate both of these problems in order to provide identificationspecificity.
Another important class of ion/molecule reactions used in ion mobility spectrometry is negative ion charge transfer or proton attraction reactions. If M is the sample molecule and R.sup.- is the reactant ion, ionization occurs in accordance with: EQU R.sup.- +M.fwdarw.R+M.sup.- (charge transfer) EQU R.sup.- +M.fwdarw.RH+(M--H).sup.- (proton abstraction)
when
M.sup.- is the product ion for charge transfer PA1 H+ is the proton PA1 (M--H).sup.- is the product ion for proton abstraction.
The tendency of an electron to transfer from the reactant ion, R.sup.-, to the sample M is regulated by the relative electron affinities of R and M. The electron affinity of M must be greater than R if the charge transfer reaction is to take place. The tendency of a proton to transfer from the sample molecule M to the reactant ion R.sup.- is regulated by the acidity of M relative to R. The acidity of M must be greater than R if the proton abstraction is to take place.
For the (H.sub.2 O).sub.n O.sub.2.sup.- or (H.sub.2 O).sub.n CO.sub.4.sup.- reactant ions conventionally used in ion mobility spectrometry, it has been shown that nitroaromatic compounds, such as mononitrotoluene (MNT), are ionizated according to: EQU (H.sub.2 O).sub.n O.sub.2.sup.- +MNT.fwdarw.MNT.sup.- +nH.sub.2 O+O.sub.2
or EQU (H.sub.2 O).sub.n CO.sub.4.sup.- +MNT.fwdarw.MNT.sup.- +nH.sub.2 O+CO.sub.2 +O.sub.2
As a result of the low electron affinity (0.44 to 0.50 electron volts) of the (H.sub.2 O).sub.n O.sub.2.sup.- and (H.sub.2 O).sub.n CO.sub.4.sup.- reactant ion's and their strong tendency to change cluster size with small variations in water concentration, two difficulties are encountered in their use as reactant ions. First, they lose charge to a large number of compounds, such as halogenated compounds, anhydrides, enols, etc. which have electron affinities greater than themselves and, second, their cluster size changes with water concentration causing them to shift in drift time in the ion mobility spectrum. Similar characteristics also exist for the proton abstraction reaction. For a microprocessor controlled ion mobility spectrometer system, it is desirable to eliminate both of these problems in order to provide identification specificity.